There has been a great deal of interest in the processing of lateritic nickel ores in the past few years, with the Western Australian projects, those of Minara Resources (formerly Anaconda Nickel) at Murrin Murrin, Preston Resources at Bulong, the Cawse plant (now owned by OMG), and BHPBilliton's Ravensthorpe Project, together with Inco's Goro Project in New Caledonia, being principal examples.
All of the purely hydrometallurgical processes developed to date for the commercial processing of lateritic nickel ores have employed a sulfate medium, following on the original process developed and operated at Moa Bay in Cuba since 1959, as described by Chalkley and Toirac (Chalkley. M. E. and Toirac, I. L., “The acid leach process for nickel and cobalt laterite. Part 1: Review of operations at Moa” in Nickel Cobalt 97, Volume 1, Hydrometallurgy and Refining of Nickel and Cobalt, I. Mihaylov and W. C. Cooper, Editors, Proceedings of the 27th Annual Hydrometallurgy Meeting of CIM, CIM, Montreal, August 1997, p 341). These processes have attempted to optimize a pressure leach process in various ways, as described by Ness and Hayward (V. H. Ness and N. L. Hayward, Nickel Laterite Processing: Second Generation Design Problems, SME Preprint 01-65, SME Annual Meeting, Denver, Colo., Feb. 26-28, 2001). However, sulfuric acid is monoprotic at the temperatures employed (240-280° C.), and therefore twice the anticipated quantity of acid has to be added to effect leaching. When the pressure in the system is let down to atmospheric pressure for further processing, the acid returns to the biprotic form and thus a substantial amount of free acid has to be neutralized. Various schemes have been investigated to overcome this difficulty, including the method proposed by BHP Minerals International in U.S. Pat. No. 6,261,527, issued Jul. 17, 2001, in which a proportion of the saprolite ore is used to neutralize the excess acid. This approach is being followed by BHPBilliton at Ravensthorpe.
The advantages generally promoted for using a pressure acid leach in the sulfate system are common materials of construction, and effective and efficient control of iron. The system is, however, inefficient in dealing with a feed that has significant magnesium values, which is a characteristic of saprolitic lateritic nickel ores. A magnesium sulfate solution is obtained, which may be crystallized and then roasted in order to recover the sulfuric acid. However, this is an expensive process, requiring both a roaster and a sulfuric acid plant to convert sulfur dioxide gas generated in the roasting step back to sulfuric acid. Sulfate processes are discussed at length by Lalancette in PCT application WO 02/08477.
Chloride flowsheets have been proposed for the treatment of lateritic nickel ores, for example as described by Gibson and Rice (Gibson, R. W. and Rice, N. M., “A hydrochloric acid process for nickeliferous laterites” Nickel Cobalt 97, Volume 1, Hydrometallurgy and Refining of Nickel and Cobalt, I. Mihaylov and W. C. Cooper, Editors, Proceedings of the 27th Annual Hydrometallurgy Meeting of CIM, CIM, Montreal, August 1997, p 247). This paper discloses leaching in hydrochloric acid, wherein a large proportion of the iron is dissolved, and recovering the iron by solvent extraction and pyrohydrolysis, following teachings practiced in the steel pickling industry. The value and tonnage of the iron products in comparison to those of nickel renders the process economically unattractive. A variation of the process is proposed by Moscony et al. in U.S. Pat. No. 5,718,874, issued Feb. 17, 1998, wherein solvent extraction is employed to separate iron and nickel values.
Demarthe, et al., in U.S. Pat. No. 4,435,368, issued Mar. 6, 1984, proposed a process wherein a suspension of feed material is treated with chlorine gas to oxidize and solubilize all of the base metals present.
Gandon et al., in U.S. Pat. No. 3,661,564, issued May 9, 1972, disclose a method of roasting a laterite ore with hydrochloric acid, followed by leaching to solubilize the chlorides of nickel and cobalt.
Canadian Patents 1023560 and 1,013,576 disclose methods of recovering nickel and cobalt from lateritic nickel ores by selective reduction of the ore followed by HCl leaching and chlorine gas treatment respectively. These processes suffer from the draw back of having to carryout a selective reduction step prior to leaching, which is energy intensive.
A process for recovering non-ferrous metal values from a metal-containing sulphide material containing at least one of zinc, copper, lead, cobalt, nickel, silver and gold, as well as iron, is disclosed in U.S. Pat. No. 4,378,275 of Adamson et al., issued Mar. 29, 1983. The sulphide material is leached under oxidizing conditions with a relatively dilute acidic aqueous chloride lixiviant solution containing magnesium chloride. The oxidizing conditions which are disclosed use molecular oxygen in the form of air, oxygen-enriched air and pure oxygen. Although leaching at atmospheric pressure is stated to be possible, it is preferable to operate the leach stage under elevated partial pressures i.e. under pressure leach conditions. Use of elevated temperatures is preferred i.e. at least about 50° C. to about 250° C., with temperatures in the range of 100° C. to 180° C. being preferred. The period for leaching is from about 5 minutes to about 12 hours. The kinetics of the process indicate a need to use very long periods of leaching at the lower temperatures and atmospheric pressure, and present applicants have verified that this is so. Pressure leaching, using oxygen, of a Zn/Cu/Fe ore containing very low levels of nickel at 160° C. is exemplified. In the process, non-ferrous metal values are solubilized, leaving iron oxide and sulphur as a residue. The iron oxide is shown to be goethite, and this is known to require elevated temperatures (i.e. above the boiling point) to have reasonable (<4 hours) rates of formation. Goethite is also notoriously difficult to handle in the subsequent solid/liquid separation step. The leach liquor is subjected to liquid-liquid extraction using a hydrophobic extractant. The raffinate, containing magnesium chloride and any sulphates formed during the leach process, is subjected to pyrohydrolysis to yield hydrogen chloride and magnesium oxide. The sulphates are then removed by washing of the magnesium oxide formed, which counteracts most of the advantages of forming magnesium oxide by pyrohydrolysis.
Duyvesteyn et al. in U.S. Pat. No. 5,571,308, issued Nov. 5, 1996, disclose a heap leach process using hydrochloric acid for the recovery of nickel from Ni—Fe—Mg laterite ores having high magnesium contents, with iron being removed by pyrohydrolysis as Fe2O3. Lalancette, in the aforementioned PCT application WO 0208477, claims that recoveries from the Duyvesteyn process are poor for nickel and cobalt. Lalancette further argues that pyrohydrolysis of the iron-magnesium chloride solution of U.S. Pat. No. 5,571,308 would produce a mixture of hematite and magnesium oxychloride. Magnesium oxychloride, is insoluble and cannot be washed out of the hematite. Further, the process of U.S. Pat. No. 5,571,308 has many steps, and is an expensive way of processing iron associated with the nickel in lateritic ores.
In PCT application WO 02/08477, Lalancette discloses a method for recovering nickel and cobalt, as well as magnesia, hematite and chromium in a chloride system. The ore is leached in very strong gaseous hydrochloric acid, and two methods are then proposed for liquor treatment and metal recovery. In the leach process, Lalancette claims that over 85% of the iron and magnesium dissolve. Iron is recovered using a modified form of spray roasting at about 200° C., and the nickel/cobalt salts then washed out of the solids. This approach is energy intensive, requires several processing steps, and has inherent problems with maintaining a water balance.
Ammonia may be used as a lixiviant for laterite, as described by Caron (Caron, M. H. “Fundamental and practical factors in ammonia leaching of nickel and cobalt ores” J. Metals 67 (1950) Trans AIME 188(1) p 91) and used commercially by, for example, Queensland Nickel in Australia and Nquel Tocantins in Brazil. However, this process requires an initial reduction roasting step to reduce metal oxides to metallic form for leaching by ammonia, which is energy intensive and provides low recovery of cobalt. A further disadvantage of ammonia is that the effluent contains nitrogen which is not environmentally acceptable. In order to reduce the nitrogen level in the effluent to acceptable levels, steam stripping may be used. However, steam stripping is energy intensive.
In summary, none of the processes developed or proposed to date are able to economically and technically handle both iron and magnesium found in lateritic nickel ores, while maintaining high metal recoveries. Sulfate-based processes require low-magnesium feeds, do not recycle the acid, and operate at high temperatures (240-280° C.) and pressures. The ammonia-based process requires an expensive pre-roasting step, and furthermore suffers from limited cobalt recovery. Any magnesium dissolved in the process is also a problem and is costly to deal with.
There are no known operating chloride-based processes for the treatment of lateritic nickel ores. A number of processes have been proposed, but these either (i) require a pre-treatment step such as roasting to render the iron relatively inert, or (ii) incur a very high dissolution of iron and consequently an expensive step to handle the dissolved iron. High levels of magnesium extraction, simultaneously with that of iron, are also produced, resulting in high acid consumption and downstream processing constraints due to the high levels of magnesium in solution.